The fate of human skin grafts has been shown to be governed in man mainly by the HLA chromosomal region. This includes many genes, some of which govern lymphocyte activation (D and DR) and others (A,B, and C) the antigens against which immunization occurs. A correlation has been found between the survival time of skin or kidney grafts and the number of A and B incompatibilities. The role of D or DR incompatibilities is still being debated as well as their mechanism of action. Skin grafts made in DR under conditions of complete compatibility, semi-incompatibility or complete incompatibility will be performed or former grafts will be re-evaluated after DR typing. The mechanism of action will be studied in vitro using the primary (MLR I), secondary (MLR II or PLT) or even tertiary proliferation (MLR III).